1. Field of the Invention
The present invention relates generally to tools used in directional drilling in earth formations and, more specifically, to a drive sub used to change the axial alignment of components of a directional drilling tool.
2. Background of the Invention
In the art of earth drilling there are two standard types of drilling, drilling a straight hole, often a vertical hole, which is commonly referred to as performance drilling. The other type of drilling is called directional drilling, which is when the path of the borehole needs to deviate from straight by some degree. The amount of deviation varies widely and the degree from deviation from straight can be different over the course of the borehole. In the prior art there are three ways which have commonly been used to accomplish directional drilling.
Early directional drilling was accomplished by placing an offset pin in a sub located at the top of the motor. This device was called an “offset top sub.” These subs allowed a user to attach and screw standard drill collars into the top of the mud motor and everything from that point up the hole would be on one common axis, which would be offset at some degree “X”. “X” could be between 1.5° to 5°, depending on what was called for in a particular drilling situation. However, in actual experience, it was found that with a 3.5° or higher bend machined into this top sub it was difficult to get the tool down hole due to the motor scraping on the sides of the hole. However, a large degree offset top sub was often needed to get any turn or reaction down hole out of the motor due to the fact that the distance between the bit and this machined offset top sub was sometimes in excess of 40 feet.
The next development in directional drilling was the fixed bent housing and/or “bent sub”, which had a fixed bend built into the middle of the device. It was found that if an offset top sub was used in conjunction with a fixed bent housing an operator could get the desired degree of offset down hole without having all of the problems of trying to get a 5° offset top sub into the hole. Later, the fixed bent sub with a bend up to 3 degrees would become the standard for modern day directional drilling. On disadvantage of this type of setup, however, was the fact that the operator only had one fixed set bend to work with and, if the direction of drilling was not being changed quickly enough, it was necessary for the operator to pull out of the hole and change mud motors with a fixed bent housing that was machined to a higher degree bend. This is undesirable due to the fact that a drilling operation might have up to four different bent housings screwed onto four different mud motors sitting at a drill site waiting to be used. The need for these extra fixed bent housing mud motors required for directional drilling operations increased the overall cost of the job.
In order to avoid having a plurality of bent subs on site, “adjustable bents subs” were developed in which the magnitude of the bend was adjustable. U.S. Pat. No. 4,077,657 to Trzecia; U.S. Pat. No. 4,813,497 to Wenzel; and U.S. Pat. No. 5,343,966 to Wenzel et al. all disclose various types of adjustable bent housings of the type under consideration. An adjustable bent housing allowed the drill operator to set the motor to whatever degree desired on the rig floor before going into the borehole. The adjustable bent housing had a bend range from 0°, or a straight motor, up to 3° of bend. With this tool, if the operator was behind the curve and consequently not building enough angle to hit the subterranean target, it was possible to pull the mud motor out of the hole, and without changing any parts, reset the degree of bend on the motor to a higher more aggressive bend. However, adjustable bent housings still cause large amounts of friction, and hence wear, on all of the parts below the bend. Both drill bits and certain parts of the mud motors, especially areas below the bent housing, are complex and expensive pieces of machinery. Reducing unnecessary wear on these parts is therefore highly desirable.
As a result, a need has continued to exist for further improvements in directional drilling equipment and methods of operation.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.